Preheat oven to 350°F. Line an 8x8 baking pan with parchment paper, leaving a little bit of extra paper on the sides of the pan. This will give you something to hold onto as you lift the cooked blondies from the pan. If you don't have parchment paper, use foil sprayed with cooking spray.

In a large bowl, add melted butter and brown sugar. Mix until creamy, about 30 seconds to 1 minute. Add egg and vanilla, mix until combined. (I use a stand mixer for this recipe because it's so fast. But an electric hand mixer or mixing exclusively by hand works well too.)

In a separate bowl, combine salt and flour. Add this flour mixture to the wet ingredients in small amounts (approximately 1/3 cup) at a time until it's all combined together.

Add the mashed bananas to the batter and mix until the bananas are evenly incorporated, approximately 30 seconds to 1 minute. Pour the batter into the prepared pan and smooth it out so that the batter is even all over.

Bake for 25-30 minutes, until the center is set.

Transfer the pan to a wire rack for cooling. After 5 minutes, lift the blondies from the pan using the parchment paper edges. Cool completely on a wire rack before frosting.

In a medium saucepan, over medium heat, melt the butter. Add the brown sugar and milk. Stir to combine and cook until the mixture comes to a boil. Stir frequently. As soon as it starts to boil, remove the pan from the cooktop. Allow to cool for approximately 5 minutes.

Stir the powdered sugar into the slightly cooled butter, brown sugar and milk mixture. Use a whisk to break apart any lumps.

Remove the Blondies from the parchment paper or foil. I like to flip the blondies over, making the bottom now the top. It makes removing the parchment paper easy. This also creates a nice flat surface to frost. Spread the warm frosting onto the cooled Blondies. Allow the frosting time to set up before cutting into servings.